Nowadays, particle-optical apparatus with focused ion beams are, for example, applied in the semiconductor industry for the purpose of processing wafers with focused ion beams. To this end, an ion source is imaged onto the wafer into a so-called ion spot. The processing speed with such ion sources is limited by the ion current density in this ion spot. A high ion current density is achieved by focusing a bright ion source into the ion spot. It is hereby desirable to use ions which do not remain behind in the processed wafers, such as noble gas ions.
The gas ion source described in said US Patent comprises a diaphragm wall, at a first side of which diaphragm wall is located a gas that is to be ionized, at a gas pressure of, for example, 0.2 bar. At the other side of the diaphragm wall is located vacuum, or at least a space with lower gas pressure. In the diaphragm wall, an exit diaphragm is fitted, through which exit diaphragm gas flows out into the vacuum. Electrons generated by an electron source at the vacuum side of the diaphragm wall are accelerated by a first electric field, the acceleration field, and focused by an electron lens, whereby the electron focus is located just before the exit diaphragm on the vacuum side of the diaphragm wall. As a result of collisions between the electrons in the electron focus and the emerging gas atoms, gas ions are now formed in an ionization volume that is thus in the direct vicinity of the exit diaphragm. The volume of the ionization volume is determined by the region in which, concurrently, a high electron density and also a high gas density occur. The ions are extracted from the ionization volume with the aid of a second electric field, the extraction field, and can then be imaged and manipulated with the aid of particle-optical means known per se.
In said US Patent, it is described how a high brightness can be obtained for the gas ion source by keeping the ionization volume small, seeing as the brightness is otherwise limited by plasma and space charge effects. The tiny dimensions of the ionization volume thus desired are achieved as a result of the fact that the exit diaphragm has a cross-section of at most 20 μm. The result of such a small exit diaphragm is that there is only a small volume at the vacuum side of the diaphragm wall where the gas pressure is so high as to result in a sizeable chance of the formation of ions. In addition, according to said US Patent, the electron beam should have a high current density.
A magnitude of the brightness cannot be derived from said US Patent, nor can the energy spread of the produced ions be derived.
Nowadays electron sources, such as sources employing field emitters, Schottky emitters or Carbon Nano Tubes, are often used when there is a need for high brightness electron sources. These sources have small electron-emitting surfaces. As known to the skilled artisan, these sources should be imaged by optics with small aberrations, especially when a relative large current in the image is to be obtained.
However, as ions emerge from the ionization volume, the optics may spatially interfere with the ions emerging from the ionization volume, or the electric fields used in the optics may interfere. As a result the focal length of the imaging optics can not be chosen arbitrarily small. As known to the skilled artisan the combination of a large focal length and small aberrations are conflicting requirements.
The sideways injection of the electrons into the ionization volume, perpendicular to the field extracting the ions from the ionization volume, aggravates the problem of obtaining small aberrations.
It is thus a problem for an ion source as described in said US patent to focus electrons from a high brightness electron source onto the ionization volume while obtaining a high electron current.
In those branches of industry in which workpieces (such as wafers) are processed with focused ion beams, there is a desire for particle-optical apparatus employing gas ion sources with a high brightness and a low energy spread. Only then the geometrical aberrations and the chromatic aberrations caused by imaging optics forming the ion spot can be made small, resulting in the desired small ion spot.